1. Field of the Invention
This invention relates to digital RF receivers, and more particularly to digital receiving systems and methods capable of simultaneously servicing multiple users.
2. Description of the Related Art
Conventional automobile radio systems have amplitude modulation (AM) and frequency modulation (FM) capabilities. Many automobiles are also equipped with cellular radios for both reception and transmission. Additional services that utilize other portions of the electromagnetic spectrum and are either currently being implemented, or are expected to be implemented in the future, include facsimile, computer, and global positioning system (GPS) services.
Reception of the various services is complicated by a desire to accommodate multiple simultaneous users. For example, it is desirable to have a capacity for cellular telephone calls at the same time the radio is playing on either AM or FM. It is also anticipated that different users will be able to plug into the communication system at the same time, such as by separate headphones assigned to different passengers, with each passenger having the ability to receive his or her own radio station while the other passengers are listening to their individual stations, using the cellular phone, etc.
The different broadcast bands are very dissimilar in terms of their bandwidths, modulation technology and band operation. The conventional approach to receiving multiple channels over multiple bands is to simply provide multiple receivers, with a separate receiver assigned to each band. If a capability for multiple simultaneous use of a single band is desired, multiple receivers are dedicated to that one band. Each additional receiver carries a penalty in terms of cost, weight, power and space requirements.
Digital receivers have been perceived as a way to accommodate very dissimilar types of modulations with a single receiver mechanism, eliminating the need for different types of receivers for each different service band. Since the channel selection frequency tuning, channel isolation and demodulation are all accomplished digitally, only a single digital receiver path is required for all of these functions. The transition between different broadcasting formats and bandwidths is accomplished by simply changing filter coefficients in the digital filters and the demodulation algorithms in a programmable demodulator. Such a system is envisioned in U.S. Pat. No. 5,058,107, by Stone et al., issued Oct. 15, 1991 and assigned to Hughes Aircraft Company, the assignee of the present invention, the contents of which patent are incorporated herein by reference. While the co-pending patent application achieves a significant reduction in system complexity and cost by using common digital computation for the different service bands, it can provide service to only one user at a time. Multiple receivers would therefore be necessary to service multiple simultaneous users.
Another digital receiver is disclosed in a paper by Dieter Baecher, "Society of Automotive Engineers Technical Paper Series", International Congress and Exposition, Detroit, Paper No. 861039, 1986, pages 77-84. An IF (intermediate frequency)-sampled, rather than RF-sampled, digital receiver is discussed. It processes only one received signal at a time; multiple IF sampled digital receivers would be necessary to process multiple signals.
A digital receiver that is capable of handling certain kinds of multiple signals simultaneously is disclosed in J. Ashjaee, "Ashtech XII GPS Receiver", IEEE International Position Location & Navigation Symposium, Nov. 28, 1988. However, this system is not applicable to common services such as FM, AM or cellular. It is designed for systems such as GPS in which all channels are broadcast at the same frequency, but are provided with different codes. The receiver processes multiple signals by code division multiplexing.
In U.S. Pat. No. 4,884,265 to Schroeder et al., assigned to Loral Corporation, a frequency division multiplexed input signal is digitized. The digitized samples are translated in frequency by mixing with baseband frequency signals to yield real and imaginary values that correspond to phase information in the original modulation signals. After translation, the samples are filtered in real and imaginary digital filters. The original modulation information is then recovered by analysis of the positions of vectors in the complex plane represented by the real and imaginary values. The translation is preferably performed by multiplying the input samples by digital values which correspond to sine and cosine values of local oscillator signals at baseband frequencies. The use of pre-select filtering prior to translation to decimate the input signals, and thereby reduce subsequent processing requirements, is suggested. While it is an improvement in digital receiver design, the system disclosed in this patent again does not resolve the problem of handling multiple simultaneous uses.